A porous honeycomb structure is widely used as a filter (diesel particulate filter (DPF)) for capturing and removing particulate substances included in a dust-containing fluid such as a diesel engine exhaust gas, or a catalyst carrier for carrying a catalyst component which purifies toxic substances in an exhaust gas. The structure is constituted of cell partition walls (ribs) forming a composite member of a plurality of adjacent cells, and a honeycomb outer wall which surrounds and holds outermost peripheral cells positioned in an outermost periphery of this cell composite member. A porous material (porous body) made of a ceramic such as fire-resistant silicon carbide (SiC) is used as a member constituting such porous honeycomb structure.
Moreover, as such honeycomb structure, there is disclosed a porous silicon carbide catalyst carrier having a honeycomb structure obtained by: using as a starting material, for example, a silicon carbide powder having a predetermined specific surface area and containing impurities, forming the material into a desired shape; drying the resultant; and firing the dried formed body in a temperature range of 1600 to 2200° C. (see, e.g., Japanese Patent Application Laid-Open No. 6-182228).
In case of the DPF, there are major demands for the improvement of permeability (reduction in pressure losses) which largely influences fuel consumption of a diesel engine provided with the filter, and improvement of a material strength which influences durability of the filter itself.
Heretofore, there has not been proposed yet any distinct theory which has been adopted in an evaluation method for predicting a permeability from characteristics of the porous body. The permeability of the porous body has been predicted and evaluated based on an empirical rule that the permeability of the porous body is proportional to “(porosity)×(mean pore size/2)2,” which is the simplest model from the mathematical standpoint. Therefore, heretofore, attempts have been made to exclusively enhance the porosity (e.g., a value actually measured by Archimedes method) and/or the mean pore size (a value actually measured with a mercury porosimeter) in order to improve a permeation property of the porous body. However, there are problems that the enhancement of the porosity of the porous body involves a strength drop and that enlargement of the pore size causes a drop of the efficiency in capturing the particulate substances (particulates) to be captured. Additionally, there is a substantial restriction to increase the respective values. The afore-mentioned proportional relationship is not always valid, depending on the porous body. Especially in a part of the porous bodies whose porosities and mean pore sizes are enhanced, the permeability cannot attain the value predicted from the proportionality relationship. One should say that a theory and an evaluation method for practical use from an engineering standpoint have been not established.
Specifically, there is a problem that the porous body cannot retain a necessary and sufficient strength during the use as a filter such as the DPF, when the porosity exceeds 80%. The enlargement of the pore size is largely promoted by making, into coarse particles, an organic pore former, or using an organic hollow pore former (e.g., foamed resin or the like) when used for manufacturing the porous body. There is a problem that the particulate capturing efficiency of the resultant porous body largely drops when the pore size exceeds 100 μm. Furthermore, since a large amount of organic pore former is used in manufacturing the porous body whose porosity exceeds 60%, a part of the pore formers is agglomerated. During the firing, ink-bottle-like coarse pores are dotted therefrom. Especially in a case where the mean pore size of such porous body is evaluated with the mercury porosimeter, there is a problem that the value of the size largely rises, and the permeability (pressure loss) is overestimated in the above-described evaluation method. That is, in the above-described conventional evaluation method, a correlation is exhibited in a part of materials constituting the porous body, but is not necessarily exhibited in other materials.
The present invention has been developed in view of such problems in the conventional art, and an object thereof is to provide a ceramic porous body for use as a member constituting a filter such as a DPF, which retains a sufficient strength while having a high porosity and which exhibits a high capturing efficiency and a high permeability, and a method for evaluating the permeability of the body.